1. Field of the Invention
The present invention relates to a single mode phosphate laser glass fiber. The single mode laser glass fiber according to the present invention is suited for use as a light source for fiber systems such as fiber optic communication, fiber optic measurement and the like.
2. Description of the Prior Art
As a single mode laser glass fiber, there is already known a single mode silica laser fiber obtained by adding a rare earth element as a laser active ion to a silica glass, as seen in pages 738-740 of Electronics Letters 15th Aug. 1985 Vol. 21 No. 17. A fiber laser oscillator using such a single mode silica laser fiber comprises (a) the silica fiber and (b) dichroic mirrors provided at the both ends of the silica laser fiber, which dichroic mirrors transmit a pumping light and have a reflectivity against a lasing light. The laser fiber is end-pumped with a pumping light emitted from a laser source such as laser diode, argon laser or the like.
A single mode glass fiber can be made easily utilizing the forming ability of a glass and can confine a light in its narrow fiber core to generate a high power density. Consequently, their laser oscillation threshold is very low. Further, owing to the end-pumping by selective wavelength, the glass fiber lasers operate at a high efficiency. The very low oscillation threshold and high efficiency operation are advantageous because a higher output can be obtained from a given input. Moreover, by being made into a fiber form, they have a large surface-area-to-volume ratio; therefore, can give a high cooling efficiency; and as a result, can provide long-term and stable operation without requiring any cooling device. Furthermore, since their connection with other fibers is easy, the glass fiber lasers are suitable as a tunable laser source of high output in fiber systems.
However, single mode silica glass fiber lasers, containing a laser active ion in a small amount of less than 900 ppm and being made of a silica laser glass, have a small stimulated emission cross section and give a small gain per unit length. Meanwhile, pages 198-199 of Electronics Letters 13th Feb. 1986 Vol. 22 No. 4 describes that by utilizing the low loss of silica glass fiber and employing a longer fiber length, there could be obtained a silica laser fiber giving a higher gain and having laser characteristics of laser oscillation threshold=2.6 mW and slope efficiency=33% at a fiber length of 3.2 m.
As mentioned above, the conventional single mode silica glass fiber lasers have needed the use of a long fiber length in order to obtain good oscillation characteristics. It is because these fiber lasers can not contain the laser active ion in an increased amount, the laser material has a small stimulated emission cross section and thus the lasers have poor laser characteristics.
In contrast, phosphate laser glasses, as compared with the silica laser glasses, have good laser characteristics but have a large coefficient of thermal expansion and show a sharp viscosity change with the change of temperature, making fiber drawing difficult. Therefore, there is known no case in which any phosphate laser glass has been made into a fiber.